Underlayment with best R value

I am going to install an engineered floor in a bedroom that is above the garage.
The room can get cold. Therefore, I want to do the best I can to help insulate while doing this project.

The regular foam underlayment appear to have very little R value. There are some with reflective foils. Seems tempting, but I am assuming it will not help much as there is not much radiant heat to "defend".

Are there any products better than others when thermal insulation is key?

I agree with Joe. Use the underlayment your flooring manufacturer suggests to keep your warranty in effect. Insulate below the floor (garage ceiling) with Roxul or other quality insulation. You don't want anything soft beyond what the manufacturer suggests. The flooring will pull part at the joints and feel squishy.

Hi destruct,
The issue of cold rooms above a garage is common, be it a bathroom or bedroom and has been discussed here often. As Joe and Larry have said, the ultimate solution lies below. Some have used electric heat in the floor to offset the heat loss due to limited insulation below. Of course I'm guessing, do you know what is down there?

Bathroom areas have a second problem in that there are often air paths beside the plumbing pipes which allow cold outside air direct access to those floors.

One more comment, don't be tempted with the exaggerated claims from those foil bubble people. If the stuff really worked as they claim we would all be wrapping our homes in it. Research has shown otherwise.

Is it insulated? You will always have a problem with cold in that area. The only solution I have seen in such areas is spray foam insulation. The cantilever is always subject to cold and heat extremes on 3 sides, and there is no warmth, period, as there is (somewhat) inside the garage part. Bud will be back to comment on that better, I'm sure.

Garage ceilings are notorious for being under insulated. They staple up some r-19 batts (or less) into a 10 or 12 inch cavity and call it good. Combine that with a lack of air sealing and you have cold floors and rooms above.

The best solution would be to have a contractor blow cellulose into that garage ceiling to completely fill that space. The cellulose will greatly reduce any air movement and add plenty of r-value.

The world would love to have a high r-value thin insulator like you are looking for, just doesn't exist, except in advertising.

Note, I would find a corner in the basement where I could remove a small, say 16" x 24" piece of drywall where I could inspect that garage ceiling. Then, if there is space above the current insulation I would get some bids for the cellulose. They install it through small holes so they don't have to remove the entire ceiling.

I've seen some strange things done where that room cantalevers over the outside wall.
Can you post a picture of the outside of the home in that general area?
Far to often I've seen where they installed blocking where the joist sit on the wall, added soffit vents, not even installed insulation for some example's.

You must realize that the garage is an unheated space even if the sidewalls. The doors (especially with a 2 car garage keep the temperature down close to the exterior because there is no heat delivered to the space. Two cars, each making a trip can require the air inside the garage from being evacuated 4 times (possibly 4 openings for a minute or two). All of the great insulation will not get the money out of the insulation and the joists are great conductors as evidenced by actual tests where the effective R-value of a floor or wall is reduced by 25% to 50% of what the insulation manufacturers claim.

If a layer of good rigid foam (1" to 2") could be applied to the bottom of the joists (requires removal of the existing sheet rock). This provides real insulation and eliminates most of the loss through the joists. It is proven and effective, but not a cheap solution and does a lot of the air sealing if the joints are taped.